The relief valve generally controls an oil pressure within a hydraulic circuit so as not to be greater than a predetermined level by opening a passage through which an excess oil in the hydraulic circuit may be discharged when the oil pressure within the hydraulic circuit reaches or is greater than a predetermined level. For example, a relief valve, disclosed in Journal of Technical Disclosure No. 2006-505946, Japan Institute of Invention and Innovation (Nov. 9, 2006), is configured by a body, a valve, a spring, a retainer, a plug, an oil passage provided so as to be connected to a portion formed between the retainer and a plug and through which the oil flows, and a back-pressure control valve for controlling the flow of the oil. According to the relief valve disclosed in Journal of Technical Disclosure No. 2006-505946, when the oil does not flow into the portion formed between the retainer and the plug, the retainer is arranged in the vicinity of an end surface of the plug, and at this point, the length of the spring becomes longest (e.g., the spring is extended), accordingly a level of pressure to open the relief valve, so that the oil sucked by an oil pump is discharged toward the upstream of the oil flow relative to the relief valve, is set to be low (e.g., a low pressure state), on the other hand, when the oil flows into the portion formed between the retainer and the plug, the retainer is moved upwardly so as to compress the spring, accordingly the level of the pressure to open the valve of the relief valve is set to be high (e.g., a high pressure state).
According to the known relief valve, an error may occur while the level of the pressure to open the valve is controlled in accordance with a driving condition. When the error occurs, the relief valve may remain its state in one of the high and low pressure states and may not be moved toward the other of the high and low pressure states. For example, according to the relief valve disclosed in Journal of Technical Disclosure No. 2006-505946, when an error occurs, for example the back-pressure control valve is clogged due to, for example sludge of the oil, the oil may not flow to the portion formed between the retainer and the plug, as a result, the pressure to open the valve may be maintained at the low level. In this condition, even when a rotational speed of the engine is high at which the oil needs to be provided in order to lubricate each part of the engine, because the level of the pressure to open the valve is set to be low, the relief valve may be easily opened so that the oil may be discharged through a drain. Accordingly, the oil may not be supplied to each part of the engine, and the engine may eventually burn out.
A need thus exists for a relief valve, which is not susceptible to the drawback mentioned above.